1. Field of the Invention
This invention relates to a position detection apparatus and a semiconductor-device manufacturing method using the apparatus. The apparatus and method will be used, for example, when projecting an electronic-circuit pattern formed on the surface of a first object (such as a reticle or the like) onto the surface of a second object (such as a wafer or the like) in a projection exposure apparatus (a stepper) for manufacturing semiconductor devices, and performing relative positioning (position detection) between the reticle and the wafer when performing an exposure transfer.
2. Description of the Related Art
In a projection exposure apparatus for manufacturing semiconductor devices, relative positioning between a reticle and a wafer is an important factor for providing high-precision devices.
Recently, in particular, submicrometer accuracy in positioning has been requested in accordance with the need for semiconductor devices having a high degree of integration.
As one of the methods for performing relative position detection (alignment) between a reticle and a wafer, there is a TTL (through the lens) method, in which the position detection is performed through a projection lens.
Various kinds of position detection apparatuses of this method have been proposed, in which relative position detection between reference positions (reference marks) on a predetermined surface and alignment marks provided on the surface of a wafer is performed using a light beam having a wavelength different from that of the exposure light used when illuminating a pattern on the surface of a reticle to print the pattern on the surface of the wafer.
The assignee of the present application has proposed,
for example, in Japanese Patent Application No. 1-198261 (1989), an observation method and an observation apparatus in which high-precision position detection can be performed while observing an alignment-mark image on a predetermined surface, for example, the surface of image pickup means (such as a CCD (charge-coupled device) camera or the like) using the image pickup means.
If the wavelength width of the light beam used in a position detection apparatus is narrow, when, for example, observing alignment marks on the surface of a wafer on which a resist is coated, many interference fringes are generated caused by light reflected by the surface of the resist and the surface of the wafer, thereby causing an error in detection.
In order to reduce such interference fringes, projection exposure apparatuses have been proposed, in which alignment marks are observed using a light source emitting a polychromatic light beam having a large spectral width whose half-width is about several tens of nanometers.
Conventional position detection apparatuses have the problem that it is difficult to identify the exact position being detected on an alignment mark on the surface of a wafer due to the relationship between the optical depth of field of the detection optical system and the form of the alignment mark produced by a given process. This problem is one of the factors which reduces accuracy in position detection.
The cause of this problem will now be described with reference to the accompanying drawings.
FIG. 1 is a schematic cross-sectional view of an alignment mark.
In FIG. 1, for the purpose of simplification, a case is illustrated in which the alignment mark is formed by etching a silicon (Si) wafer, and a photoresist (PR) is coated on the surface of the wafer.
When focusing the wafer surface onto the surface of an image pickup device, such as a CCD or the like, by a detection optical system, if it is assumed, for example, that the wavelength .lambda. of the light beam being used is 632.8 nm, and the numerical aperture of the system on the wafer surface is NA=0.5, the optical depth of field expressed by .+-..lambda./(2 NA.sup.2) becomes.+-.1.3 .mu.m (having a range of 2.5 .mu.m).
In most of the current semiconductor manufacturing processes, both the step d of the alignment mark and the thickness T of the resist equal at least about 1 .mu.m.
As shown in FIG. 1, illuminating light produces various kinds of reflected light, such as reflected light L1 reflected by the photoresist surface PRF, refracted light L2 deflected by being refracted at an inclined portion of photoresist PR, scattered light L3 scattered at an edge portion of the alignment mark, and the like. An alignment-mark image is formed on the surface of the image pickup device of the CCD camera by all of such reflected light.
If the refractive index of photoresist PR is represented by N, the geometrical optical length equals L+d/N, which is the same order as the optical depth of field.
The alignment-mark image on the surface of the image pickup device is formed by reflected light from the photoresist surface PRF and reflected light from the wafer surface. Hence, it is impossible to identify the surface being detected.
FIG. 2 illustrates a case in which the shape of the cross-section of an alignment mark becomes asymmetrical due to the wafer-forming process.
As described above, the detection optical system detects reflected light from all surfaces, i.e., the photoresist surface PRF, the wafer's top surface WF and the wafer's bottom surface WB, without discriminating among respective light beams. Hence, it is impossible to focus the system on the wafer's bottom surface WB if it is desired to do so.
FIG. 3 illustrates a case in which multiple transparent layers are provided on the wafer surface in the process.
An alignment mark LOCOS is formed by a local oxidation of silicon process, and a PSG (phosphosilicate glass) layer is formed thereon. At that time, if the shape of the cross-section of the structure becomes asymmetrical due to inferior coverage of the PSG layer, the obtained image signal becomes asymmetrical. Hence, even if the LOCOS mark is symmetrical (having no error) and it is desired to perform alignment with the LOCOS mark, accuracy in detection is reduced due to the asymmetry of the PSG layer.
If photoresist PR is coated in an inferior manner, the photoresist PR becomes asymmetrical in the vicinity of the alignment mark even if the lower portion of the alignment mark is symmetrical, as shown in FIG. 4(A). Hence, the waveform of the video signal 0S.sub.v from the image pickup means becomes asymmetrical, as shown in FIG. 4(B).
As a result, accuracy in position detection of the alignment mark is reduced as in the above-described cases. The waveform of the video signal becomes asymmetrical due to various factors, such as asymmetry in the pattern on the wafer, in a transparent or translucent film provided thereon, in a resist, and the like. These factors cannot be neglected when high accuracy in positioning is required.